<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    html body{
        height: 100px;

    }
      #a{
          width: 100%;
          height: 100%;
          background-color: pink;
      }
      #b{
          width: 50%;
          height: 50%;
          background-color: orange;
      }
      #c{
          width: 50%;
          height: 50%;
          background-color: green;
      }
</style>
<body>
    <div id="a">
        <div id="b">
            <div id="c"></div>
        </div>
    </div>
</body>
</html>